1. Field of the Disclosure
The present invention relates generally to power supplies, and more specifically to switched-mode power supplies.
2. Background
Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency and good output regulation to power many of today's electronics. Typically, a control circuit included in the switched mode power supply converts a low frequency (e.g. 50 or 60 Hz mains frequency), high voltage alternating current (ac) to high frequency (e.g. 30 to 300 kHz) ac. This high frequency, high voltage ac is applied to a transformer to transform the voltage, usually to a lower voltage, and to provide safety isolation. The output of the transformer is rectified to provide a regulated dc output, which may be used to power an electronic device. The switched mode power supply control circuit usually provides output regulation by sensing the output and controlling it in a closed loop. In operation, a switch is utilized to provide the desired output by regulating the duty cycle (typically the ratio of the on time of the switch to the total switching period) of the switch in a switched mode power supply.
Requirements such as efficiency, size, weight and cost are usually taken into account when designing a switched mode power supply. Greater efficiency may often be achieved by reducing the various losses which exist in a power supply for any input voltage. Typical losses experienced in a power supply are conduction losses and switching losses. Conduction losses and switching losses occur due to the electrical resistance in the circuit and the parasitic capacitance that is switched by the power supply, particularly when the switch is a transistor. When the switch conducts current, the resistance of the circuit along with the current passing in the circuit generates conduction loss. When the switch is open, voltage across the switch stores energy in the parasitic capacitance. The parasitic capacitance discharges when the switch closes, dissipating the energy stored in the parasitic capacitance in the resistance of the switch to produce switching loss. It is generally understood that a switch that is closed may conduct current and is considered on, while a switch that is open does not conduct current and is considered off. As the input voltage into the power supply increases the switching losses may increase due to increased voltages which are switched.
Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention.